Continuous casting roll for casting molten baths and method for producing one such continuous casting roll

ABSTRACT

The present invention relates to a casting roller for casting molten metal, particularly molten steel, having a roller body ( 2 ) made of a metallic material, onto which a coating ( 4 ) is applied, which is produced from a metal material having a greater hardness than the material of the roller body ( 2 ). The casting roller according to the invention has an increased service life and improved usage properties. This is achieved in that the coating ( 4 ), viewed over its thickness (D), has a lower hardness in its region bordering the roller body ( 2 ) than in the region of its free surface ( 5 ).

[0001] The invention relates to a casting roller forecasting molten metal, particularly molten steel, having a roller body made of a metallic material, onto which a coating is applied which is made of a metal material having a greater hardness than the material of the roller body. Casting rollers of this type are used in machines for continuous casting of molten steel into cast strip.

[0002] For strip casting of steel in such devices, which are also referred to as “double-roller casting machines”, two casting rollers are provided, which counter rotate during the casting process, are positioned with their axes parallel, are internally cooled, and which delimit the long sides of a casting gap implemented between them. Sufficient liquid molten metal is poured into this casting gap each time so that a molten metal pool forms above the casting gap. The molten metal which reaches the casting rollers from this molten metal pool solidifies and is conveyed by the casting rollers into the casting gap. In the casting gap, the cast strip, which is subsequently drawn off and supplied for further processing, is shaped from the shells thus formed on the casting rollers and the still free-flowing molten metal.

[0003] In order to ensure good thermal conductivity of the casting rollers, they are typically made, at least in the region of their lateral surfaces, of a copper alloy. In this way, good dissipation of the heat is achieved. However, in practical operation the casting rollers are subject to strong mechanical and thermal strains, which lead to significant wear in the copper material used for manufacturing the casting rollers.

[0004] For this reason, casting rollers used for casting steel are typically provided with a coating on their lateral surface which has a greater hardness than the other material of the casting roller. A method of producing the coating on the roller body of a casting roller, which roller body is made of copper, is known from EP 0 801 154 B1. According to the known method, the lateral surface of the casting roller, which is made of a copper material, is coated with a nickel coating while paying attention to specific method steps, so that the finished casting roller is coated with an abrasion-resistant, hard coating on its lateral surface which comes into contact with the molten metal. This nickel coating protects the copper roller from mechanical damage and reduces its thermal strain. Simultaneously, there is a higher temperature in the region of the coating, due to its poorer thermal conductivity, than in the region of the roller body, which is made of copper, which has better thermal conductivity. An improved surface of the cast strips is achieved through the higher temperature of the outer coating.

[0005] A problem in the casting rollers provided with a coating of the type described above is that the particular coating applied exhibits different thermal expansion behavior than the copper material of the roller body which supports it. If these differences are too great, cracking or spalling of parts of the coating may occur. The section of the roller body then exposed comes directly into contact with the molten metal, which causes additional wear.

[0006] To solve this problem, applying two different coatings onto a roller body made of copper, the coating applied directly to the roller body expanding more strongly upon heating than the external coating, which comes directly into contact with the molten metal, has been suggested in EP 0 421 908 B1. In this way, the external coating is to be tensioned on the internal coating, although the internal coating is colder than the external coating due to the cooling of the roller body. In this case, the external coating is preferably made of steel, nickel, or chromium, while the internal coating is manufactured from copper. This type of double coating has the disadvantage that in this case an additional boundary layer is provided between the first and the second coating, at which adhesion problems may occur under unfavorable process conditions or due to non-uniform material composition. Therefore, in practical operation, there are also problems in regard to the reliability and abrasion resistance of the coating according to EP 0 421 908 B1.

[0007] The object of the invention is, starting from the related art described above, to provide a casting roller which has an increased service life and improved usage properties. In addition, a method of manufacturing such a casting roller is to be specified.

[0008] This object is achieved for a device of the type described above in that the coating applied to the roller body, viewed over its thickness, has a lower hardness in its region bordering the roller body than in the region of its free surface.

[0009] According to the present invention, only one single coating, preferably based on a nickel alloy, is applied to the roller body. However, this coating is composed in such a way that it has a lower hardness in the region bordering the roller body than in the region of its external surface, which comes directly into contact with the molten metal. As a consequence of this hardness, which increases outward over the thickness of the coating, starting from the roller body, the coating has increased ductility in the region adjoining the roller body, so that it may absorb the deformations of the copper jacket of the roller body in this region and good adhesion of the coating on the roller body is ensured. The danger of the coating detaching from the roller body in operation is prevented in this way, so that long-lasting, reliable operation of the casting roller is ensured. Simultaneously, the hardness at the circumference of the casting roller is so high that the surface of a casting roller according to the invention has good wear resistance and therefore has an elevated service life.

[0010] The varying hardness trend in the coating applied to the roller body may be produced in that the coating has a different composition in its region adjoining the roller body than in the region of its free surface. Thus, for example, in the course of the coating, the proportions of those elements in the particular electrolytes used may be increased, which lead to elevation of the hardness of the particular material deposited. In this way, a composition of the alloy which varies over the thickness of the coating is obtained. Its distribution is implemented in such a way that it is less hard and simultaneously more ductile in the region adjoining the roller body than at the circumference of the casting roller. In this context, sodium citrate or sulfur, for example, are suitable as hardness-increasing elements, which may preferably be used if the coating is produced on the basis of a nickel alloy.

[0011] Alternatively or additionally to influence the hardness trend by changing the composition, the coating process may also be controlled in such a way that the coating has a softer microstructure in its region bordering the roller body than in the region of its free external surface. This may be achieved in that the depositing current is increased in the course of the coating process, starting from a minimum value. Through a low depositing current strength at the beginning of the coating, a fine-grained coating thickness region is first implemented, which has increased ductility. Through subsequent increase of the depositing current strength, the microstructure becomes coarser and the hardness of the coating rises with increasing thickness.

[0012] The coating process is to be controlled in each case such that a homogeneous coating structure is obtained, which has the particular optimum material properties in the particular problem zones (boundary layer between the roller body and the coating; external surface of the coating). This may be ensured by adding the hardness-increasing elements to the electrolyte or changing the depositing current in each case such that the hardness increases continuously over the thickness of the coating. Depending on the particular coating material used, its thickness, and the particular operating influences, it may be expedient in this case if the hardness has a linearly or progressively increasing trend starting from the circumference of the roller body.

[0013] Practical experiments have shown that casting rollers according to the present invention reliably meet the demands posed on them when the hardness of the coating is 150 HV to 250 HV in the region boarding the roller body and is 400 HV to 500 HV in the region of the free surface. This is particularly true if the coating is produced based on a nickel material.

[0014] Further advantageous embodiments of the invention are specified in the dependent claims and are described in greater detail in connection with the following exemplary embodiment, which is described on the basis of a drawing. FIG. 1 shows a casting roller in a perspective view, FIG. 2 shows the casting roller in cross-section in an enlarged detail.

[0015] The casting roller 1 has a roller body 2, in which at least the outer lateral surface 3 is made of a copper material. Inside, the roller body 2 is equipped with cooling devices (not shown), via which it is cooled in casting operation.

[0016] A coating 4, which is based on a nickel alloy, is applied to the lateral surface 3 of the roller body 2 through electrolytic deposition. In order to produce a hardness trend in the coating 4 which increases linearly starting from the lateral surface 3 over the thickness D of the coating 4, indicated by horizontally running dashes in FIG. 2, the strength of the depositing current has been increased continuously during the coating process. In this way, the coating 4 was formed slowly at the beginning of the coating process, so that in its region which comes into contact with the lateral surface 3 of the roller body 2, the coating 4 has a fine microstructure and a low hardness, in the region of 150 to 250 HV, while simultaneously having increased ductility. Therefore, the coating in the concerning thickness region may follow the expansion of the roller body 2 which results because of the heating during casting.

[0017] In the course of the coating process, the depositing current has been continuously increased linearly, with the consequence that the coating formation occurs more and more rapidly and with increasingly coarse grain. In this way, the hardness of the coating increases linearly in the direction of the external surface 5, which comes into contact with the molten metal, so that the coating has a maximum hardness in the range from 400 to 500 HV, which causes high wear resistance, at the external surface 5.

Reference Numbers

[0018]1 casting roller

[0019]2 roller body

[0020]3 external lateral surface of the roller body

[0021]4 coating

[0022]5 external surface

[0023] D thickness of the coating 4 

1-21. (canceled).
 22. A casting roller for casting molten metal, particularly molten steel, having a roller body made of a metallic material, onto which a coating is applied, which is made of a metal material having a greater hardness than the material of the roller body, the coating, viewed over its thickness, having a lower hardness in its region bordering the roller body than in the region of its free surface, wherein the hardness of the coating increases continuously beginning with the region bordering the roller body up to its free surface.
 23. The casting roller according to claim 22, wherein the coating has a different composition in its region bordering the roller body than in the region of its free surface.
 24. The casting roller according to claim 22, wherein the coating has a softer microstructure in its region adjoining the roller body than in the region of its free surface .
 25. The casting roller according to claim 22, wherein the hardness increases linearly.
 26. The casting roller according to claim 22, wherein the increase of the hardness has a progressively increasing trend.
 27. The casting roller according to claim 22, wherein the hardness of the coating is 150 HV to 250 HV in the region bordering the roller body and is 400 HV to 500 HV in the region of the free surface.
 28. The casting roller according to claim 22, wherein the roller body is manufactured from a copper alloy, at least in its lateral surface, which is bonded to the coating.
 29. The casting roller according to claim 22, wherein the coating is formed based on a nickel alloy.
 30. A method of manufacturing a casting roller in which a coating made of a metallic coating material is applied to a roller body through electrolytic deposition, wherein the deposition current is increased starting from a minimum value in the course of the coating process.
 31. The method according to claim 30, wherein the depositing current is increased continuously.
 32. The method according to claim 31, wherein the increase is linear.
 33. The method according to claim 31, wherein the increase is progressive.
 34. The method according to claim 30, wherein the coating is formed based on a nickel alloy.
 35. A method of manufacturing a casting roller, in which a coating made of a metallic coating material is applied to a roller body through electrolytic deposition, wherein in the course of the coating process, the proportions of the elements in the electrolytes which lead to an increase of the hardness of the particular material deposited are increased.
 36. The method according to claim 35, wherein the increase of the proportions of elements which increase hardness takes place continuously.
 37. The method according to claim 36, wherein the increase takes place linearly.
 38. The method according to claim 36, wherein the increase takes place progressively.
 39. The method according to claim 35, wherein the coating is formed based on a nickel alloy.
 40. The method according to claim 35, wherein sodium citrate is added to the electrolyte as hardness-increasing element.
 41. The method according to claim 35, wherein sulfur is added to the electrolyte as hardness-increasing element. 